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"On-Approach" Logic on STAR?

Started by emerydc8, Sat, 1 Aug 2015 10:56

emerydc8

It's complicated enough when you are descending straight towards the crossing fix. But when you are flying away from it in some parts of the STAR, I suspect it makes it even more complicated. Maybe this was a bad arrival to use to test this.

So, if it was looking at MURRY at FL130 (or between FL110 and FL130) and projecting back towards towards the top of descent, shouldn't it be able to do a 3 degree path without any knees getting in the way? All of the waypoints going backwards from MURRY to the top of descent are FL130A, so they are really no more restrictive than MURRY at FL130.

This is so confusing! I wish Peter or others could give us their input here. I have to step out for a few hours, but I should be back at it later today. I want to try some different arrivals that aren't as circuitous!

Britjet

#21
Quote from: emerydc8.......you can speed intervene and it will stay in VNAV PTH. That's not supposed to happen unless you are in on-approach logic.
I agree.
I am pretty sure that you can only get SPD and VNAV PTH when flying level at an FMC altitude constraint, or when on approach logic.

Otherwise, "intervening" should give VNAV SPD...

Maybe something amiss here?

Peter

emerydc8

#22
I apologize if I'm adding more confusion with this post, but I did more research and wanted to get it out here so that we can have a better discussion of what the FMS should be doing.

First, I wanted to distinguish situations where you can speed intervene and still remain in VNAV PTH and situations where you don't speed intervene (speed window is closed) but you see |SPD|LNAV|VNAV PTH| on the FMA during a VNAV descent.

I agree with Peter that the only time you can speed intervene and stay in VNAV PTH is when flying level at an FMC altitude constraint, or when in on-approach logic.

I did, however, find three situations on a descent where you can see |SPD|LNAV|VNAV PTH| with the speed window closed:

1) If after the first altitude constraint waypoint, the second VNAV path segment is too shallow to be flown in IDLE, then you will see |SPD|LNAV|VNAV PTH| while descending on that segment.

2 & 3) If there is a flight plan modification or unknown winds when above the first speed constraint, VNAV will slow up to 15 knots below the FMC target speed to stay on path. If that's not enough, the autothrottle will change from IDLE/HOLD to SPD. Here's a link to the two pages I'm using for reference:

https://drive.google.com/file/d/0B-WRh0Hf7VdZM3Jud19VOXA3Mms/view?usp=sharing

I tried a new situ today going into JFK on the LENDY 6 Arrival. https://drive.google.com/file/d/0B-WRh0Hf7VdZM3Jud19VOXA3Mms/view?usp=sharing

To make it simple, I didn't insert the FL230 at JENNO. I just set up one crossing restriction at LENDY (FL190 and 250 knots). You can see for yourself how it does. Actually,  if you do nothing, it comes pretty close to making the altitude and speed. It crosses LENDY at about FL196 and 265 knots. It does get 1500' high when slowing to 250 about 12 miles from LENDY, but it seems to close up the gap by the time it gets there.

As for the FMA, at top of descent it correctly goes to |IDLE/HOLD|LNAV|VNAV PTH|. And if you speed intervene, it correctly goes to |HOLD|LNAV|VNAV SPD|.

On a VNAV PTH descent, there are two ways that I know of to get into VNAV SPD -- speed intervention or getting more than 150' away from the path. During the descent in the situ above, it correctly went to VNAV SPD when I speed intervened; but when the aircraft was more than 150' from the path (above or below), it incorrectly displayed VNAV PTH instead of VNAV SPD.

Since it went above the path on its own in my situ, I purposely put it in a position below the path to test the FMA here as well. As soon as it starts descending out of FL370, speed intervene and select M.90. It will correctly go to |HOLD|LNAV|VNAV SPD| and start diving below the path (keep PROGS 2/3 open to see how far below). When it gets at least 1000' below the path, close the speed window. The displayed FMA -- |HOLD|LNAV|VNAV PTH| -- is incorrect. It should display |HOLD|LNAV|VNAV SPD| until getting back within 150' of path.

Once you close the speed window, it will revert back to the FMC target descent speed and in doing so it will reduce the rate of descent to 1200 FPM. This is the path capture feature of VNAV which is basically the same thing it does if you start an early descent. If power is also needed to maintain the 1200 FPM descent, it should display |THR/HOLD|LNAV|VNAV SPD| until within 150', then it should go to |IDLE/HOLD|LNAV|VNAV PTH|.

So, while the actual descent profile seems to be pretty close when using just one crossing altitude and speed restriction (HKG was probably not a good place to start), the FMA's do not seem to be indicating correctly when more than 150' from the path.

I also noticed that the speed variations available to capture or stay on path were limited to only about 8 knots. According to the attached material, prior to reaching the first speed constraint (in this case LENDY), the FMC should have accelerated up to Vmo - 16 (349 knots) before the DRAG REQUIRED message was displayed on the scratchpad. Likewise, if below the path, the aircraft should have slowed to FMC target speed -15 before adding thrust.

When I put the airplane 1000' below the path, it added thrust in an attempt to re-acquire the path before it slowed to this speed . When thrust was added, the FMA incorrectly indicated |THR|LNAV|VNAV PTH|. It should have indicated |THR|LNAV|VNAV SPD| because it was more than 150' below the path.

Britjet

#23
That is a very comprehensive manual that you have, Jon. I have never seen such detail and was certainly not aware of many of the nuances...

There seems to be two aspects to this, apart from the approach logic discussion, which I think we all agree on. They are whether we should see THR or SPD, and whether we should see VNAV SPD or VNAV PTH. One caveat is we should never see SPD-VNAV SPD!

Peter

emerydc8

#24
Yes, but SPD-V/S will not cause the wings to fall off the airplane like some guys fear!

Maybe another aspect would be the amount of airspeed deviation from FMC target speed to stay on path.

Hardy Heinlin

#25
I've learned that the allowed speed "flexibility" is -10/+15 knots when above speed transition altitude, else -10/+10 knots.

-16 knots sounds a bit critical to me. Is this value from an official source, Jon? You could drop into the lower part of the lift/drag curve and so the conditions regarding recapturing the path from below could get even worse. Additional drag is fine when above the path, but not when below the path, I would say.


|-|ardy



Quote from: BritjetThere seems to be two aspects to this, apart from the approach logic discussion, which I think we all agree on. They are whether we should see THR or SPD, and whether we should see VNAV SPD or VNAV PTH. One caveat is we should never see SPD-VNAV SPD!
I agree ...

Britjet

Quote from: emerydc8Yes, but SPD-V/S will not cause the wings to fall off the airplane like some guys fear!

Of course not, Jon, that is very silly of them. The THR button does that..

Peter

emerydc8

#27
Quote-16 knots sounds a bit critical to me. Is this value from an official source, Jon?

Yes, Hardy. This is right from my airline's systems manual (Vol. 2) and it probably came from Boeing.

The speed increase from FMC-target speed to stay on path is up to Vmo - 16 (349 knots). This gives a lot more ability to stay on path. You shouldn't see the DRAG REQUIRED scrathcpad message until it reaches 349 knots and it still can't keep it within 150' of the path. That's when speed reversion occurs and it goes to VNAV SPD.

You run into the smaller deviations once you are below the first speed constraint (see page 2).

QuoteOf course not, Jon, that is very silly of them. The THR button does that.

I try to tell them that!

Hardy Heinlin

#28
I mean minus 16 sounds critical. Plus 16 is OK.

DRAG REQUIRED relates to the positive side.

I agree that +16 is correct.

-10 and +15 are the limits.

+16 is an exceedance of +15 :-)

Below 10000 feet +15 becomes +10, i.e. +11 is then the exceedance. You shouldn't exceed 250 KIAS below 10000 feet.

emerydc8

Just so we're on the same page here (page 1), the 16 refers to Vmo minus 16 (354 knots) which is how fast it should be able to go to maintain the path if you are being pushed above it on descent. The 15 refers to 15 knots below target FMC, which is how slow it should be able to go to maintain the path if you are falling below it.

emerydc8

#30
Quote from: Hardy HeinlinI mean minus 16 sounds critical. Plus 16 is OK.

DRAG REQUIRED relates to the positive side.

I agree that +16 is correct.

-10 and +15 are the limits.

+16 is an exceedance of +15 :-)

Below 10000 feet +15 becomes +10, i.e. +11 is then the exceedance. You shouldn't exceed 250 KIAS below 10000 feet.

I think 10,000 would normally be the first speed constraint unless there is a speed constraint higher than 10,000 -- like on the LENDY 6 (250 knots at LENDY). After that point, it should go to the second set of rules on page 2. So, 10,000 would be the speed transition altitude, but not necessarily the speed constraint point.

Britjet

#31
Just to be clear...the speed does not increase to 349 every time and then  a "drag required" message appears. That's daft. The only time it will fly that fast without giving you a message is if you put a very high speed into the FMC.

The accepted + increments apply on whatever speed you put in the FMC. NOT 349 kts blindly...no way...

(Descent at 350 kts is crazy anyway IMHO)

Peter

emerydc8

I'm having trouble with that one. If you put 280 knots in the FMC, then got pushed high prior to the first speed constraint, when would you get the DRAG REQUIRED message and how fast would the aircraft go to stay on path?

Britjet

#33
10 kts until you get the message, Jon,

You aren't seriously trying to tell me that if you put 280 kts in the box it won't tell you that you need drag until you get to 349?

Peter

emerydc8

#34
Where are you getting that 10 knot figure?

I'm just reading from the FCOM:

"[W]ith greater than 349 knots (VMO/MMO minus 16 knots) the scratchpad message DRAG REQUIRED displays. The airplane may accelerate up to 354 knots (VMO/MMO minus 11 knots) to maintain the path."

Britjet

From your own text, page 2, about para 3  ( sorry, I don't seem to be able to copy and paste).

10 kts for the message, 15 kts for the allowed speed deviation.

I have seen the 349 kts text as well. Like a lot of Boeing publications the context can be lost or misunderstood. I don't think ( in fact I know tbh) that this is not the general case.

When did you ever fly a 747 that let the speed get to 349 kts before it gave you a drag message?

emerydc8

#36
What you are referring to on page 2 is in contexts where you are below the first speed constraint. If you are above it, I think page 1 applies and the 349/354 would be used.

I'm trying to find anything in the manuals that would controvert page 1 because, like you, I too think I have seen the DRAG REQUIRED message while above the first speed constraint and I was not doing 349 knots.

Britjet

#37
I understand what you are saying, Jon, and I understand the text.
But in my experience, it doesn't happen.
There is no way that you put 280kts in the box, for example, and it gets to 349 on its own...

I  don't think I have ever done 349 kts in my life for real  in any case - (the wings fall off, as you know) but I have seen plenty of DRAG REQUIRED messages...

Peter

emerydc8

#38
I agree. In practice, I think it would take one hell of an unforeseen tailwind to even get close to requiring 349 knots on a descent. I'm thinking of a situation where ATC keeps you high and you are finally released for the slam-dunk arrival. Does it wait until 349 knots to give you the DRAG REQUIRED warning? I think it happens before that. But the manual is contra. Sure wish we had access to that sim!

Come to think of it, if ATC kept you high or you just didn't reset MCP altitude and you were in cruise (M.84), I think you would eventually get DRAG REQUIRED at some point. Thoughts?

emerydc8

#39
Okay, I found this in the Big Boeing Book. I don't know if it applies to the 744, but it does tend to support your position that it will only descend up to a certain speed above the FMC target speed to stay on path.

I'd sure like to learn what Boeing meant by its reference to the 349 knots to set off the DRAG REQUIRED and up to 354 to maintain path. Above the first speed constraint, it makes no reference to the FMC target speed when the aircraft is being pushed high -- just that it will vary the speed up to 354 knots to maintain path. Do you think this could have been a software option or a later change?

QuoteDES

LATE DESCENT

Depending on where the aircraft is in the descent, there are two procedures employed by the FMS in attempting to recapture the descent path.

First Procedure: Occurs prior to the aircraft reaching any descent airspeed or altitude constraint. When above the descent path and prior to the first constraint, the aircraft will maintain the FMS target speed plus 15 kts as long as that speed is less than or equal to Mmo/Vmo. If this speed is greater than that, then the DRAG REQUIRED message is displayed.

Second Procedure: Occurs after the aircraft reaches the first constraint or is below the SPD TRANS altitude, or is in Speed Intervention. In this procedure, the aircraft maintains the FMS target plus 10 kts (or MCP speed if in S.I.), as long as that speed is less than or equal to Vmo. If this speed is greater than Vmo, DRAG
REQUIRED is displayed.